This publication includes papers of an interdisciplinary conference in 2011 analysing multilevel governance problems of the international trading, environmental, development and rule-of-law systems as interdependent ‘aggregate public goods’. It begins with policy-oriented analyses from leading practitioners on the ‘gap between theory and practice’ in multilevel governance. The analyses of the ‘collective action problems’ are supplemented by case-studies on the world trading, environmental, development and related rule-of-law systems. Apart from assessing existing multilevel governance arrangements (such as the G20 meetings), the conference papers explore alternative strategies for rendering multilevel governance more effective, for instance by promoting public-private partnerships, empowering private actors through stronger cosmopolitan rights, enhancing legal-judicial accountability of governments and by promoting overall coherence through transnational rule-of-law systems. The papers explore emerging principles for multilevel political governance (like ‘responsible sovereignty’, unilateral protection of ‘common concerns’) as well as new forms of multilevel judicial governance strengthening transnational rule-of-law in international trade, investment and environmental regulation.